Treatment of molten light metals



March 9, 1965 P. D. HESS ETAL 3,172,757

TREATMENT OF MOLTEN LIGHT METALS Original Filed Nov. 21, 1958 2 Sheets-Sheet 1 FIG. 2 PAUL 0. HESS KENNETH J. BRONDYKE 4 NOEL JARRETT INVENTORS ATTORNEY March 9, 1965 P. D. HESS ETAL TREATMENT OF MOLTEN LIGHT METALS 2 Sheets-Sheet 2 Original Filed Nov. 21, 1958 PAUL D. HESS KENNETH J.BRONDYKE NOEL JARRETT FIG. 3

INVENTOIE A TTORNEV United States Patent O 3,172,757 TREATMENT OF MOLTEN LIGHT METALS Paul D. Hess, Arnold, Kenneth J. Brondyke, Oakmont, and Noel Jarrett, New Kensington, la, assignors to Aluminum Company of America, Pittsburgh, Pa, a corporation of Pennsylvania Continuation of application Ser. No. 775,628, Nov. 21, 1958. This application May 4, 1962, Ser. No. 193,897 9 Claims. (Ci. 75-67) This invention relates to the fiuxing and filtering of molten light metals to remove gas and finely-divided nonmetallic particles therein, and more particularly to a method for such treatment employable during molten metal transfer.

The term light metal, as used herein, refers to aluminum, magnesium, and to alloys thereof wherein these metals predominate.

In the melting of light metals and their transfer to other receptacles, gas is frequently entrapped or dissolved within the molten metal. This is primarily hydrogen, most of which is probably generated by the reaction with the metal of even small amounts of moisture in the surrounding environment. Also, a film generally forms on the surface of the molten metal which largely consists of the oxides of the light metal and/ or its alloying constituents, and this film is broken up and dispersed within the molten metal during subsequent agitation or transfer. Upon casting of the metal article, a considerable amount of gas and oxide particles are entrapped within.

In the cast article, a large proportion of the hydrogen is usually considered to be in solution in the solid metal, i.e., it is in the monatomic state, although pockets or voids filled with molecular hydrogen have been observed. In the subsequent fabrication of wrought articles, thermal treatments are generally employed to aid in working the metal or to develop the desired strength, and it is generally considered that such heating produces diffusion of the monatomic hydrogen to any voids or discontinuities Within the metal whereat association into molecular hydrogen takes place. It is also believed that the small occluded oxide particles tend to nucleate the formation of hydrogenfilled voids. Recently the problem of so-called flakes within the internal metal structure of wrought products has been associated with these hydrogen-filled voids.

Because of the gas pressures developed by the molecular gas, subsequent working of the metal does not effect a healing of the void or discontinuity, and heating of the article at elevated temperatures may increase such pressures to the point where the metal suffers local plastic deformation.

The problem of occluded gas has become increasingly important with the growing requirements for high-strength light metal articles. Any gas-filled void may not only constitute an area of weakness in the final article, but may give rise to flakes, blisters, slivers and other defects which may cause rejection. This recognition has prompt ed investigations to find a method for the removal of gas from the molten metal so as to produce a substantially gas-free article.

It has heretofore been proposed to flux molten metals with chlorine, nitrogen, and inert gases to reduce the hydrogen content thereof, usually in the ladies or in the melting and holding furnaces. However, large quantities of gas generally remain in the cast article, and the process has been costly and time consuming.

It has also been proposed to filter molten aluminum through a bed of refractory filter medium to remove finely divided non-metallic solids. This procedure has proven highly beneficial in this respect but has failed to appreciably reduce the gas content of the metal. Originally, increases in vacuum density of the castings were consid "ice ered as reflecting removal of gas butmore recent methods of gas determination have indicated that this assumption was not completely correct and that the results were not quantitatively determinative of the gas remaining in the metal.

It is an object of this invention to provide a method for the substantial removal of gas from molten light metals.

It is also an object to provide a rapid method for the simultaneous removal of gas and finely divided solids during the treatment of molten light metals.

A further object is to provide a treatment for simultaneous removal of gas and solid impurities which is employable in the molten metal transfer line at high metal flow rates.

Other objects and advantages will be evident from the following description and the attached drawing wherein:

FIG. 1 is a section of an apparatus installed in a molten metal transfer trough for the practice of this invention.

FIG. 2 is a top sectional view of the refractory chamber along the line 11-11 shown in FIG. 1, in the absence of molten metal and refractory material so as to expose the inert gas device.

FIG. 3 is a section of another apparatus suitable for use in a casting downspout.

It has now been discovered that gas and non-metallic solids may be substantially removed from molten light metals by a method in which the molten metal is passed through a bed of refractory filter medium in co-current contact with an inert gas, and the inert gas is thereafter vented from the molten metal under a protective atmosphere.

The co-current flow of inert fluxing gas and molten metal should be through a distance of at least two inches of filter medium, and preferably six inches or more, and the particles of the refractory filter medium should be substantially of a granule size of 3 to 14 mesh. However, the total depth of refractory filter medium should be at least six inches.

The refractory filter medium is a substance which is inert toward the molten metal being treated. It must also have a higher melting point, possess high hardness and be of sufficient density to gravitationally remain in place during operation. Among such substances are chromite, corundum, forsterite, magnesia spinel, periclase, silicon carbide and zircon. Of these tabular alumina (synthetic corundum) is preferred. All of these materials, with the exception of forsterite and zircon, are free from silica; but in the case of the last two, the silica is chemically combined with another oxide in such a manner that it is not attacked by the molten light metal. For this reason, all of these materials are regarded as being inert towards the molten light metal.

The fluxing gas must be substantially inert towards the molten light metal. The inert gases of the periodic table, helium, neon, argon, krypton and xenon and mixtures thereof are preferred for the practice of this invention. However, nitrogen may also be employed in the treatment of aluminum, and those aluminum base alloys where nitride-formation is not of significance. Chlorine, a common fiuxing gas, has been found unsuitable for this application because of its tendency to form chlorides as well as to result in clogging of the filter medium after only short periods of operation.

An hourly metal to gas flow rate of about 30 to 500 pounds of metal per cubic foot of inert gas (at 70 F. and 760 mm.) has been found effective. A lesser amount of gas effects insuflicient gas removal whereas greater amounts do not increase the removal sufficiently to economically warrant use and may often interfere with the operation of the apparatus.

To prevent generation of oxides after the fluxingfiltering treatment, it is essential that the inert gas be vented off the metal under aprotective or non-oxidzing atmosphere. This maybe accomplished by providing a quiescent zone in which the gas may vent'off under either atmospheric or reduced pressure, such as for example in the castingmold. Another method is to merely reduce the metal flow rate suificiently to permit ready escape of theinert gas. A further method. is to provide a baffled venting zone in which the metal fiows over weirs thus exposing smallvolumes of'metal to the venting atmosphere.

The non-oxidizing or protective atmosphere is conveniently generated or provided by the inertgases venting off the metal, i.e;, by hooding or enclosing the venting zone. Duringi the initial period'of operation, however, it may bedesirable to provide the inert or non-oxidizing atmosphere from an outside source.

The inert gas may be induced to flow co-currentlywith the metal by several methods. The simplest is to use a high metal fiow rate through the bed so that the'velocity of the metal sweeps the gas along with it. Alternatively, a positive pressure of the gas may be provided abovethe filter bed and/ or a reduced pressure may be provided on the discharge side'of the filter bed.

The flow rate of metal through the bed of refractory filter medium will be determined by the head of molten metal or metal pressure, depth of the bed, fiuxing gas fi'ow rate and mode of preparation ofthe filter bed. In the present-method, improvement in filtering efiiciency and metal'fiow rate has been noted when the filter. bed is prepared by adding the refractory material to an initial body of molten metal in-the filtering container and al-' lowing the'refractory to settle therein. This method of preparation has been notably advantageous in apparatus of the type illustrated in FIG. 1 wherein the metal flow is U-shaped. More particularly, the refractory material is washed, dried and preheated to a temperature of about 1200 to 1800 F., and preferably above 1400 F., after which it is added to the filtering container which has been initially provided with molten metal sufficient to cover the refractory material which is added. thereto. The refractory is allowed to settle and may be tamped lightly. Thereafter, molten metal is passed therethrough. In this method of preparation, the fiuxing gas should be introduced prior to commencement of metal flow to degas the initial metal body of metal. Additional information concerning this method'of preparation can be found in the copending application of K. J. Brondyke and P. T; Stroup, Serial No. 655,746, filed April 29, 1957, now Patent No. 2,863,558.

The filter medium may also be prepared by heating the refractory and adding it-to the dry filter container or the dry container with the refractory bed in place may be heated to the desired temperature; in either instance, the temperature of the unit should be high enough to pre-' vent chilling and freezing of the molten metal which is subsequently introduced.

The device for introducing the fluxing gas into the filter bed may be of any of the well-known media, such as porous or perforated pipes and plates. In practice, porous carbon diffusers and perforated metal rings have been found highly satisfactory.

A base for the refractory filter medium is preferably prepared by the use of larger ceramic bodies, such as alumina balls, having a diameter of inch or more. This permits rapid flow of the metal under the baffle of FIG. 1, or rapid discharge from the apparatus of FIG. 3, wherein it also serves as a physical retainer for the finer ceramic bodies of the filter medium. The larger refractory particles are considered to have little, if any, effect upon the gas removal or upon the removal of solid impurities.

During thefiltration operation, it is essential that the apparatus be maintained at a temperature between about 1200 to 1600 F. to prevent freezing of the molten metal, and also to maintain the molten metal level above the top of the refractory material so as to prevent cohesion of the refractory mass which is now quite wet with molten metal and impurities.

It has'also been found that the co-current flow of inert gas in the refractory filter medium not only does notimpair thefiltering action of the filter medium in removing finely divided solids, but, in actuality, tends to improve it. The present invention removes the dissolved hydrogen much more thoroughly than conventional iluxing techniques. Much less fiuxingi gas is required, and a combined fluxing-filtering action is performed during metal transfer, thus saving the time required for furnace fluxing as conventionally employed. Furthermore, themethod is easily employed in the treatmenttof large volumes of metal, thus permitting its use in general casting practice.

Furthermore, the present invention. has enabled substantial lowering of the gas content of metal that had beenv previously fluxed with chlorine for considerable periods of time in the holding furnace in accordance with conventional practice.

Results have proven this method far more effective than either therefractoryfilter or fluxing gas alone. If the filter alone is used finely divided solids are removed but gas content is substantially unchanged. If gas fiuxing alone is used, often there is produced an undesired turbulence in the metal which often results in the entrapment and/or generation of finely divided solids.

That the filter medium of the present invention exerts a very great infiuence on the efficiency of gas removal has been determined by comparison to the results obtained by use of conventional packing materials. Apparently, in the diffusion of gas from the molten metal.into the fiuxing gas, a surface phenomenon is involved which limits the diffusion of the hydrogen from the metal into the inert gas bubble, such as a film at the interface between the metal and the fiuxing gas bubble.

This surface phenomenon theory has been postulated because of the variance in efficiency of the various gases in conventional iiuxing techniques. Although all gases (free from hydrogen) should theoretically permit diffusion of hydrogen from the melt at an equal rate, chlorine has proven much more effective in furnace and crucible fiuxing. Since there is no reaction between the chlorine and hydrogen, it has been suggested that the chlorine overcomes some film or surface phenomenon.

In the method of the present invention, the inert gases have proven to be far more effective than equivalent amounts of the normally superior chlorine. This would indicate that fiuxing the molten metal in accordance'with the present invention has the effect of reducing or eliminating the retarding film and permits rapid-diffusion of hydrogen into the inert gas bubbles, possibly because the solid impurities have been the interferring agency.

Referring now to FIGS. 1 and 2, therein illustrated is one form of apparatus suitable for the practice of the present inveniton. The refractory chamber or crucible 2 is partitioned by the transverse bafile 4 to provide a fiuxing section 6 in which there is situated a fiuxing gas device 8, which may be a porous or perforated manifold, and which is connected to the inert gas supply by the entry tube 1%.. The device illustrated is a metal ring which has perforations 9 about the bottom. Alternatively, severalporous or perforated tubes may be employed to introduce the gas into the bed, or the gas may be fed into the space above the molten metal and bed under pressure. In the device illustrated, a layer of large refractory particles 12 has been placed at the base of the refractory chamber, and the finer ceramic bodies 14 constituting the refractory filter-medium have been deposited thereon.

Molten metal from the furnace flows under the baffle 18 into theapparatus from the inlet trough 20, passes downwardly through the filter medium 14 wherein it is fiuxed by the co-current flow of inert gas from the fluxing gas device 8. It then passes under the baffle 4 and upward in the venting section 22. The inert gas escapes in the venting section 22 and is discharged through the gas vent 24. The baflie 26, under which the metal must pass, to enter the outlet trough 27, serves to maintain an inert gas atmosphere above the molten metal in the venting zone and the bafiie 28, over which the metal must flow, aids the venting of the inert gas.

TABLE 1.Hydr0gen-s0lids removal by inert gas-filter 1 Vacuum densit (London),

y tests may be made in apparatus of the type described in Lights Metals vol. 15, pages 306, 307 (Sept. 1952). The molten metal is frozen under a vacuum of 5 mm. and the density determination is made on the casting.

2 Hydrogen determinations made by the Telegas instrument described in the Journal of the Institute of Metals (London), vol. 86, pp. 212-219 (1958).

ency of the gas to rise will thus provide co-current contact with the metal in the refractory bed.

Referring now to FIG. 3, illustrated is a downspout apparatus suitable for co-current fiuxing of the metal. The cylindrical casing 150 is provided with an overflow discharge 152 and is inserted into the molten metal trough 154 wherein the flanges 156 of the casing 150 are carried by the collar 158 which is afiixed to the metal form 1613 of the transfer trough. Also illustrated is the refractory lining 16?. generally provided in the metal carrying equipment. The gas vent hood 164 is provided with legs 166 and the gas vent pipe 168, and the entire assembly may be easily removed as a unit. The fluxing gas ring 170 may be provided with perforations 172, or it may be constructed of porous material, such as porous carbon, and is affixed to the inert gas entry tube 174, thus facilitating removal.

To prepare the filter bed, large refractory material 176 such as alumina balls, is placed at the base of the filter chamber and the small refractory particles 178 which form the filter medium are placed thereon. The fluxing gas ring 176 is inserted into the apparatus after the desired depth of filter medium has been attained and additional refractory material is superposed. The metal head or hydrostatic pressure of the molten metal 180 tends to determine the rate of flow through the refractory bed.

In the operation of the unit, molten metal passes downwardly through the filter bed of refractory material 178 in co-current contact with the inert gas from the gas ring 170. As the metal flows under the hood 164 and into the discharge 152, the inert gas vents off the metal and is carried off by the vent pipe 168. The venting gas provides an inert atmosphere in the hood 164 but the diameter of the vent pipe 168 must be sufficiently small that the venting gases will prevent the entry of any oxidizing gases therethrough.

Indicative of the efiicacy of the present invention are the data presented in Table 1, wherein a unit substantially as illustrated in FIG. 3 was used in the treatment of an alloy nominally composed of aluminum, 4.4 per- As is evident from the data in the preceding Table, the method of the present invention is highly effective in removing gas from the molten metal while simultaneously eliminating finely divided, non-metallic solids. This method is very rapid, adaptable to use in conventional metal trough systems, and relatively inexpensive in comparison to the general practice of fluxing in the melting furnace. By the method of this invention, a greatly superior product can be obtained, both as cast and after subsequent working operations and attendant thermal treatments.

This application is a continuation of our co-pending application Serial No. 775,628, filed November 21, 1958, now abandoned.

Having thus described the invention, we claim:

1. In the treatment of molten light metals for the substantial removal therefrom of gas and finely divided solids, the method comprising providing a container through which the molten light metal is passed, said container having a filter bed therein composed of refractory granules 3 to 14 mesh in size and inert toward the molten light metal, completely covering said bed with molten metal, introducing metal to be treated above the bed and passing it downwardly therethrough and at the same time introducing an inert fluxing gas into the downwardly flowing molten metal and within said filter bed both metal and fluxing gas flow in co-current relationship with each other through at least two inches of said bed, passing said metal with fluxing gas therein to a zone having a non-oxidizing atmosphere and releasing said fiuxing gas and the gas derived from the molten metal in said zone.

2. In the treatment of molten light metals for the substantial removal therefrom of gas and finely divided solids, the method comprising providing a container through which the molten light metal is passed, said container having a filter bed therein composed of refractory granules 3 to 14 mesh in size and inert toward the molten light metal, completely covering said bed with molten metal, introducing metal to be treated above the bed and passing it downwardly therethrough, at the same time passing an inert fluxing gas downwardly through at least a portion of the bed and in co-current relationship with the descending molten metal for a distance of at least two inches in the bed, passing said metal with timing gas therein to a zone having a non-oxiding atmosphere and releasing said fluxing gas and the gas derived from the molten metal in said zone.

3. The method according to claim 2 wherein the proportion of inert fiuxinggas introduced to the molten metal being treated is one cubic foot of fiuxing gas to 30 to 500 pounds of molten metal.

4. The method according to claim 2 wherein the refractory granules are preheated to a temperature between 1200 and 1800 F. and added to a body of molten metal in the container before filtering is commenced.

5. The method according to claim 2 wherein the inert fiuxing gas is introduced adjacent the top of the filter bed.

6. The method according to claim 2 wherein the filter bed of 3 to 14 mesh size refractory. granules is supported upon coarser refractory bodies.

7. Themethod according to claim 2 wherein the treated metal isdischarged from the container and the spent inert fiuxing gas and that gas derived from the. molten metal are released from the molten metal in a nonoxidizing zone below the filter bed.

8. The method according to claim 2 wherein the zone having, a non-oxidizing atmosphere is under reduced atmospheric pressure.

9. The method according to claim 2 wherein the rate of metal flow is reduced in the zone having a non-oxidizing atmosphere.

References Cited in the file of this patent UNITED STATES PATENTS 1,972,432 Girsewald Sept. 4, 1934 1,994,358 Holstein Mar. 12, 1935 2,821,472 Peterson Jan. 28, 1958 2,863,558 Brondyke Dec. 9, 1958 FOREIGN PATENTS 297,086 Great Britain Sept. 11, 1928 684,048 Great Britain Dec. 10, 1952 1,040,447 France May 20, 1953 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,172,757 I March 9,, 19 65 Paul D. Hess et a1;

It is hereby certified that error appears :in the above ent requiring cerrectio'n and that the said Letters Patent should rehdl le corrected below.

Column 3, Tine 3, for "non-oxiglzing" read non-oxidizing column 6 llne 52 after "bed" lnsert so that line 73, for non-oxiding" read non-oxidizing Signed and sealed this 27th day of July 1965 (SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Aitcsting Officer Commissioner of Patents 

1. IN THE TREATMENT OF MOLTEN LIGHT METALS FOR THE SUBSTANTIAL REMOVAL THEREFROM OF GAS AND FINELY DIVIDED SOLIDS, THE METHOD COMPRISING PRIVIDING A CONTAINER THROUGH WHICH THE MOLTEN LIGHT METAL IS PASSED, SAID CONTAINER HAVING A FILLER BED THEREIN COMPOSED OF REFRACTORY GRANULES 3 TO 14 MESH IN SIZE AND INERT TOWARD THE MOLTEN LIGHT METAL, COMPLETELY COVERING SAID BED WITH MOLTEN METAL, INTRODUCING METAL TO BE TREATED ABOVE THE BED AND PASSING IT DOWNWARDLY THERETHROUGH AND AT THE SAME TIME INTRODUCING AN INERT FLUXING GAS INTO THE DOWNWARDLY FLOWING MOLTEN METAL AND WITHIN SAID FILTER BED BOTH METAL AND FLUXING GAS FLOW IN CO-CURRENT RELATIONSHIP WITH EACH OTHER THROUGH AT LEAST TWO INCHES OF SAID BED, PASSING SAID METAL WITH FLUXING GAS THEREIN TO A ZONE HAVING A NON-OXIDIZING ATMOSPHERE AND RELEASING SAID FLUXING GAS AND THE GAS DERIVED FROM THE MOLTEN METAL IN SAID ZONE. 